The application relates to filterless coherent optical receivers.
Coherent optical detection is becoming a mature commercial technology and is revolutionizing optical fiber communications. Amongst the advantages of coherent detection are improved receiver sensitivity, increased spectral efficiency, and digital compensation of channel impairments, which are enabled by recovery of the electric field. The intermarriage of coherent detection with tunable optics is facilitating dynamically tunable transceivers whose operations are defined by control layer software. We recently proposed a colorless and directionless reconfigurable optical add/drop multiplexer (ROADM) architecture that uses coherent receiver, where a local oscillator (LO) laser is tuned near the center frequency of the channel of interest for demodulation, in each transponder among all dropped channels at the node. This architecture does not require wavelength selectors, such as optical demultiplexer or tunable filter array, at the transponder aggregator, and therefore the receiver is called filterless receiver.
The front-end of a coherent receiver is an optical hybrid combining the signal with the LO. After square-law photodetection, the output photocurrent consists of a desired signal-LO beating term plus undesired interferences arising from signal-signal and LO-LO beating. The LO-LO beating is a DC term which can be easily rejected using a DC block. The signal-signal interference however, will occupied the same down-converted bandwidth as the signal-LO beating term and will scale linearly as the number of WDM channels increases. For example, with a typical LO-to-signal (per optical channel) power ratio of 20 dB, the signal-signal interference will reach comparable power as the signal-LO beating term when the number of WDM channel approaches 100. Therefore in the filterless receiver design, one major task is to keep the signal-signal interference term as small as possible so the loss of performance is acceptable.
Signal-signal interference may be suppressed with balanced detection, where a pair of identical photodiodes is illuminated with the signal mixed with opposite phases (180 degree difference) of the LO. The interference, being a common term in the two inputs of the photodiodes regardless of the phase of the LO, can therefore be cancelled after subtracting the output photocurrents. System performance in this case can be improved by increasing the common-mode rejection ratio (CMRR) of the optical front-end, which is determined by factors such as responsivity matching of the photodiodes, power imbalance in the optical hybrid, and timing skews between the two inputs. Replacing single-ended photodiodes with balanced photodiodes could raise the component cost for coherent receivers, while designing CMRR>20 dB also takes extra engineering efforts which may increase system complexity.
Noise and interference reduction can also be achieved by adding redundant signal mixing paths in the coherent receiver. By using 3×3 couplers instead of 90 degree hybrid as the mixing components, one can minimize the influence of noise and interference in coherent detection because of these common terms will be suppressed during the extraction of I and Q components. This approach however will increase implementation cost and complexity because of the additional channels needed for the down-conversion (three channels instead of two in each polarization). It may also require more DSP resources due to the extra steps for I and Q component extraction.